WO2013061667A1 - 浸炭検知方法 - Google Patents
浸炭検知方法 Download PDFInfo
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- WO2013061667A1 WO2013061667A1 PCT/JP2012/070159 JP2012070159W WO2013061667A1 WO 2013061667 A1 WO2013061667 A1 WO 2013061667A1 JP 2012070159 W JP2012070159 W JP 2012070159W WO 2013061667 A1 WO2013061667 A1 WO 2013061667A1
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- carburization
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/72—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables
- G01N27/82—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
- C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
- C23C8/20—Carburising
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
- C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
- C23C8/20—Carburising
- C23C8/22—Carburising of ferrous surfaces
Definitions
- the present invention relates to a method for detecting the presence or absence of carburization on the inner surface of a pipe by an electromagnetic inspection method such as an electromagnetic induction inspection method or a leakage flux inspection method.
- carburization occurs in austenitic stainless steel.
- a cracking tube used for a pyrolysis reaction in an ethylene production process of a petrochemical plant is made of austenitic stainless steel, and carburization occurs on the inner surface when used for a long time.
- carburization occurs by performing a heat treatment in a state where the degreasing failure of the lubricating oil is not performed. Since the occurrence of such carburization is a factor that greatly reduces the life of the cracking tube, it is desired to accurately detect the presence or absence of carburization.
- the cracking tube installed in the plant is subjected to electromagnetic inspection such as electromagnetic induction inspection as a non-destructive inspection over the entire length of the cracking tube during periodic repair of the plant, and the output value is small or large.
- electromagnetic inspection such as electromagnetic induction inspection as a non-destructive inspection over the entire length of the cracking tube during periodic repair of the plant, and the output value is small or large.
- the presence or absence of carburization is detected.
- the presence or absence of carburization is detected by performing an electromagnetic inspection over the entire length or by observing the microstructure by cutting both ends.
- This invention is made in view of such a prior art, and makes it a subject to provide the carburization detection method which can also detect the fine carburization difficult to detect with the conventional carburization detection method.
- the inventors of the present invention proposed a ferrite on the outer surface of a tube having fine carburization on the inner surface as described in Japanese Patent Application Laid-Open No. 2010-197222 proposed by the present inventors.
- Meters were placed facing each other, and the magnetic strength (amount of ferrite) at the carburized site was measured with this ferrite meter, but an effective indication value was not obtained.
- the magnetic strength was measured at 10 locations on the tube where it was confirmed by microstructural observation that fine carburization had occurred on the inner surface, and the indicated values of the ferrite meter were all 0.01 Fe% or less. It was. The reason why the magnetic strength is small in this way is presumed to be because the amount of magnetic oxide produced by carburization is small.
- the present inventors first tried to detect fine carburization from the inner surface rather than from the outer surface of the pipe. Specifically, a test was conducted to confirm whether carburization detection was possible under the following conditions (1) to (3) using a general flaw-inspection insertion coil.
- the detection signal (absolute value signal) output from the interpolation coil was amplified and synchronously detected to separate and extract the first signal component and the second signal component whose phases are different from each other by 90 °. Then, the phases of the first signal component and the second signal component are rotated (shifted) by the same predetermined amount, and the first signal component after the rotation is the X signal and the second signal component after the rotation is the Y signal.
- the amount of rotation is such that when the X and Y signals are represented on the XY vector plane, the Y-axis direction of the XY vector plane corresponds to the lift-off fluctuation of the tube.
- the axial direction was determined so as to correspond to the magnetic variation of the tube.
- Inspected object 13 steel pipes with an outer diameter of 19 mm and an inner diameter of 17 mm with fine carburization on the inner surface
- Interpolated coil outer diameter 16.5 mm, length 2 mm, impedance 50 ⁇ / 100 kHz
- a magnetic tape having the number of turns of 2.5 turns and 6 turns is applied to the inner surface of the steel pipe of the same type as the object to be inspected and not carburized, and detection signals obtained from these magnetic tapes are also described above. And evaluated in the same manner.
- FIG. 1 is a diagram showing the results of the above test (X signal and Y signal represented on the XY vector plane).
- the data plotted with white diamonds is obtained from the carburized portion to be inspected, and the data plotted with black diamonds is obtained from magnetic tape.
- the flaw inspection using the interpolated coil as in the above-described test detects a change in electrical resistance due to the flaw and is generally sensitive to magnetic fluctuations because a high-sensitivity inspection is performed.
- the X signal becomes a negative value corresponding to the magnitude of the magnetic fluctuation (data is plotted in the negative direction of the X axis).
- the data obtained from the carburized portion to be inspected has a positive value, and carburization cannot be detected.
- the data indicated by the arrows A, B, and C in FIG. 1 are negative values, even the data indicating the negative value having the largest absolute value (data indicated by the arrow A) has 6 turns.
- the data obtained from the magnetic tape and the X signal have the same magnitude and are only very weak magnetic fluctuations.
- the excitation capability (magnetic field strength) used is weak. That is, the magnetization characteristic of the magnetic material is represented by a BH curve, and the initial permeability when the magnetic field strength is small is extremely small, and the magnetic permeability increases as the magnetic field strength increases. For this reason, it has been found that the interpolated coil used in general flaw inspection cannot detect fine carburization that causes only weak magnetic fluctuations. In order to detect minute magnetic fluctuations, it is preferable to employ a mutual induction method in which an excitation coil and a detection coil are separately provided. However, when using an insertion coil, the size of the coil inserted into the tube is limited.
- the present inventors have reexamined a method for detecting the presence or absence of carburization on the inner surface of the pipe to be inspected. Specifically, first, using the method shown in FIG. 1 of the Japanese Patent Application Laid-Open No. 2010-197222 proposed by the present inventors (hereinafter referred to as the conventional method), the inner surface of the tube under the following conditions: The possibility of detecting magnetic tape affixed to the tape was examined. In addition, since the magnetic fluctuation
- the magnetic strength (the amount of ferrite) of the magnetic tape affixed to the inner surface of the tube was measured with a ferrite meter.
- the conventional method could not detect a magnetic tape having 3 or fewer turns.
- minute carburization cannot be detected because weak magnetic fluctuations cannot be detected under the above-described conditions. Therefore, the inventors of the present invention have a method for detecting the presence or absence of carburization on the inner surface of the tube from the outer surface of the tube, and the influence of excitation capability (magnetic field strength) and excitation frequency on the detection capability of fine carburization (weak magnetic fluctuation). Further studies were conducted as follows.
- the magnetic field strength is further increased, the magnetic flux density is saturated and the magnetic permeability is decreased. For this reason, it is difficult to detect a weak magnetic fluctuation unless an appropriate magnetic field strength is given.
- the magnetic permeability is small, the change in the output signal (output voltage) of the detection coil accompanying the magnetic fluctuation is small, so that the weak magnetic fluctuation cannot be detected.
- the detection capability of fine carburization depends on the excitation capability (magnetic field strength) in terms of maximizing the magnetic permeability.
- the penetration depth has a generally positive correlation with the excitation frequency of ⁇ 1 ⁇ 2 power, and the sensitivity (electric noise) of the signal processing unit has a negative correlation with the excitation frequency (in other words, Therefore, it was found that the detection capability of fine carburization depends on the -3/2 power of the excitation frequency.
- the present inventors have determined that the current value of the excitation current flowing through the excitation coil is I (A), the length of the excitation coil is L (mm), the number of turns of the excitation coil is N, and the excitation When the frequency of the exciting current flowing through the coil is F (kHz), the parameter K represented by the following equation (1) is considered to be an index of carburization detection capability.
- K (I ⁇ N / L) ⁇ F -3/2 (1)
- FIG. 2 is a diagram showing an example of test results obtained by investigating the relationship between the detection signal obtained from the magnetic tape attached to the inner surface of the pipe where carburization has not occurred and the parameter K using the mutual induction method.
- the horizontal axis in FIG. 2 indicates the parameter K, and the vertical axis indicates the detection signal.
- the value of the parameter K was changed by changing the conditions (excitation current, etc.) of the exciting coil 11 using the eddy current inspection apparatus 100 described later with reference to FIG.
- a detection signal (specifically, an absolute value signal output from the detection coil 12 is obtained by signal processing corresponding to the value of each parameter K and obtained from a magnetic tape having one and three turns.
- the value of the (X-axis signal) was evaluated. As shown in FIG.
- the parameter K can be an index of carburization detection capability.
- the inventors have found that fine carburization can be detected by appropriately adjusting the value of the parameter K.
- the present invention has been completed based on the above findings of the present inventors. That is, the present invention includes the following first step and second step.
- First step A carburizing material known to be carburized on the inner surface of the tube is inserted into the exciting coil and the detecting coil, and the current value of the exciting current energized to the exciting coil is set to I (A), When the length of the exciting coil is L (mm), the number of turns of the exciting coil is N, and the frequency of the exciting current energized to the exciting coil is F (kHz), the carburizing is performed based on the output signal of the detecting coil.
- the value of the parameter K represented by the following formula (1) is determined so that carburization occurring in the material can be detected.
- Second step After setting the conditions of the exciting coil so that the value of the determined parameter K is obtained, the tube to be inspected is inserted into the exciting coil and the detecting coil, and the detecting coil The presence / absence of carburization on the inner surface of the pipe is detected based on the output signal.
- the value of the parameter K is determined so that carburization of the carburized material can be detected.
- the parameter K is proportional to the magnetic field strength (I ⁇ N / L) and proportional to the ⁇ 3/2 power of the excitation frequency F.
- the parameter K expressed by the equation (1) is an index representing the carburization detection capability. I can say that. Therefore, in order to detect fine carburization, prepare a carburized material that has undergone fine carburization, determine the value of parameter K so that this carburization can be detected, that is, adjust the carburization detection capability. Good.
- the second step after setting the excitation coil conditions so that the value of the parameter K determined in the first step is obtained, the presence or absence of carburization on the inner surface of the tube to be inspected is determined. Detected.
- the condition to be inspected is set after setting the excitation coil conditions so that the value of the parameter K is obtained. If the pipe is inspected, it can be expected that the carburization equivalent to the carburization occurring in the carburized material used for determining the value of the parameter K can be detected.
- the value of the parameter K is preferably set to 4 ⁇ K ⁇ 8. That is, in the second step, it is preferable to set the excitation coil condition so that the value of the parameter K satisfies 4 ⁇ K ⁇ 8.
- the carburization detection method it is possible to detect fine carburization that is difficult to detect by the conventional carburization detection method.
- FIG. 1 is a diagram showing the results of a test conducted by the present invention using an insertion coil.
- FIG. 2 is a diagram showing an example of test results obtained by investigating the relationship between the detection signal obtained from the magnetic tape attached to the inner surface of the pipe where carburization has not occurred and the parameter K using the mutual induction method.
- FIG. 3 is a schematic diagram showing a schematic configuration of an eddy current inspection apparatus used in the carburization detection method according to the embodiment of the present invention.
- FIG. 4 is a schematic diagram showing the X and Y signals output from the phase rotator provided in the eddy current inspection apparatus shown in FIG. 3 on the XY vector plane.
- FIG. 1 is a diagram showing the results of a test conducted by the present invention using an insertion coil.
- FIG. 2 is a diagram showing an example of test results obtained by investigating the relationship between the detection signal obtained from the magnetic tape attached to the inner surface of the pipe where carburization has not occurred and the parameter K using the mutual induction method.
- FIG. 3
- FIG. 5 is a diagram showing an example of test results obtained by investigating the relationship between detection signals obtained from a plurality of carburized materials and the parameter K using the eddy current inspection apparatus shown in FIG.
- FIG. 6A is a diagram showing the results of examining the relationship between the detection signal and the carburization depth by extracting data of 4 ⁇ K ⁇ 8 from the data shown in FIG.
- FIG. 3 is a schematic diagram showing a schematic configuration of an eddy current inspection apparatus used in the carburization detection method according to the embodiment of the present invention.
- the eddy current inspection apparatus 100 of this embodiment includes a detection sensor 1 and a signal processing unit 2.
- the detection sensor 1 is shown in cross section.
- the detection sensor 1 is configured to induce an eddy current by applying an alternating magnetic field to the steel pipe P and to detect an eddy current induced in the steel pipe P.
- the detection sensor 1 of the present embodiment is a single detection that detects an excitation coil 11 that applies an alternating magnetic field to the inserted steel pipe P and an eddy current induced in the inserted steel pipe P.
- a coil 12 is a single detection that detects an excitation coil 11 that applies an alternating magnetic field to the inserted steel pipe P and an eddy current induced in the inserted steel pipe P.
- the signal processing unit 2 energizes the detection sensor 1 with an alternating excitation current, and detects the presence or absence of carburization on the inner surface of the steel pipe P based on the detection signal (absolute value signal) output from the detection sensor 1. It is configured. Specifically, the signal processing unit 2 of the present embodiment includes an oscillator 21, an amplifier 22, a synchronous detector 23, a phase rotator 24, an A / D converter 26, and a determination unit 27.
- the oscillator 21 supplies an alternating excitation current to the detection sensor 1 (specifically, the excitation coil 11 of the detection sensor 1). Thereby, as described above, an alternating magnetic field acts on the steel pipe P, and an eddy current is induced in the steel pipe P.
- the absolute value signal output from the detection sensor 1 (specifically, the detection coil 12 of the detection sensor 1) is amplified by the amplifier 22 and then output to the synchronous detector 23.
- the synchronous detector 23 synchronously detects the output signal of the amplifier 22 based on the reference signal output from the oscillator 21. More specifically, the first reference signal having the same frequency as the excitation current supplied to the detection sensor 1 from the oscillator 21 toward the synchronous detector 23 and the phase of the first reference signal are expressed as follows. A second reference signal shifted by 90 ° is output. Then, the synchronous detector 23 determines from the output signal of the amplifier 22 a signal component having the same phase as the phase of the first reference signal (first signal component) and a signal component having the same phase as the phase of the second reference signal (second signal). Component). The separated first signal component and second signal component are output to the phase rotator 24, respectively.
- the phase rotator 24 rotates (phase shifts) the phases of the first signal component and the second signal component output from the synchronous detector 23 by the same predetermined amount, for example, the first signal component is the X signal,
- the two signal components are output to the A / D converter 26 as Y signals.
- the X and Y signals output from the phase rotator 24 are called so-called Lissajous waveforms used for flaw inspection or the like on the XY vector plane represented by two axes (X axis and Y axis) orthogonal to each other.
- Signal waveform that is, the absolute value signal waveform of the detection sensor 1 expressed in polar coordinates (Z, ⁇ ) where the amplitude is Z and the phase is ⁇ (more precisely, the absolute value signal waveform after being amplified by the amplifier 22)
- Z, ⁇ the absolute value signal waveform of the detection sensor 1 expressed in polar coordinates (Z, ⁇ ) where the amplitude is Z and the phase is ⁇ (more precisely, the absolute value signal waveform after being amplified by the amplifier 22)
- FIG. 4 is a schematic diagram showing the X and Y signals output from the phase rotator 24 on the XY vector plane.
- the reference material is inserted into the detection sensor 1 and stopped so that the X signal is 0 and the Y signal is at a predetermined voltage (for example, 4 V) (the tip of the vector is positioned at the reference point shown in FIG. 4).
- a predetermined voltage for example, 4 V
- the amplification factor of the amplifier 22 and the phase rotation amount of the phase rotator 24 are adjusted.
- the steel pipe P to be inspected is moved in the axial direction and inserted into the detection sensor 1, whereby the X signal and the Y signal are acquired.
- the A / D converter 26 A / D converts the output signal of the phase rotator 24 and outputs it to the determination unit 27.
- the determination unit 27 is based on the output data of the A / D converter 26 (that is, digital data obtained by A / D converting the X signal and the Y signal, hereinafter referred to as X signal data and Y signal data). Detects the presence or absence of carburizing. As shown in FIG. 3, the position of the tip of the vector fluctuates according to the magnetic fluctuation of the steel pipe P, but the fluctuation amount is larger in the X-axis direction than in the Y-axis direction. For this reason, the determination part 27 of this embodiment detects the presence or absence of carburization using X signal data among the input X signal data and Y signal data.
- the determination unit 27 of the present embodiment compares the input X signal data with a threshold value determined and stored in advance, and if the X signal data exceeds the threshold value, It is determined that carburization has occurred on the inner surface of the steel pipe P. If the X signal data is within the threshold value, it is determined that carburization has not occurred on the inner surface of the steel pipe P.
- the carburizing material P0 which is known to have fine carburization on the inner surface, is previously applied to the exciting coil 11 and The detection coil 12 is inserted (see FIG. 3). Then, the value of the parameter K expressed by the following equation (1) is set so that carburization occurring in the carburized material P0 can be detected based on the output signal from the detection coil 12 (specifically, X signal data). Make a decision.
- I is the current value (A) of the exciting current that is passed through the exciting coil 11
- L is the length (mm) of the exciting coil 11
- N is the number of turns of the exciting coil 11
- F is the exciting coil.
- 11 means the frequency of the excitation current energized to (kHz).
- the conditions of the exciting coil 11 (current value of exciting current, length of exciting coil, number of turns of exciting coil, frequency of exciting current) are set so that the value of the parameter K determined as described above can be obtained.
- the steel pipe P to be inspected is inserted into the excitation coil 11 and the detection coil 12, and the presence or absence of carburization on the inner surface of the steel pipe P is detected based on the output signal (specifically, X signal data) from the detection coil 12. To do.
- FIG. 5 is a diagram showing an example of a test result obtained by investigating the relationship between the detection signal obtained from the plurality of carburized materials P0 and the parameter K using the eddy current inspection apparatus 100 under the following test conditions.
- the horizontal axis indicates the parameter K
- the vertical axis indicates the detection signal.
- the value of the parameter K was changed by changing the conditions of the exciting coil 11 of the eddy current inspection apparatus 100.
- detection signals specifically, X-axis signals obtained by performing signal processing on the absolute value signals output from the detection coil 12
- the value was evaluated.
- FIG. 5 is a diagram showing an example of a test result obtained by investigating the relationship between the detection signal obtained from the plurality of carburized materials P0 and the parameter K using the eddy current inspection apparatus 100 under the following test conditions.
- the horizontal axis indicates the parameter K
- the vertical axis indicates the detection signal.
- the X signal has a negative value, so that it is possible to detect magnetic fluctuations caused by the carburized portion on the inner surface of the carburized material P0.
- the magnetic field strength on the inner surface of the carburized material P0 becomes small because the current value of the exciting current is small, or because the exciting frequency is high and the penetration depth is shallow.
- the magnetic permeability of the carburized material P0 becomes small, and the magnetic fluctuation accompanying carburization cannot be detected with high accuracy.
- the threshold value stored in the determination unit 27 is set to 0, for example, and then the steel pipe P to be inspected is inspected, the steel pipe P It can be said that the presence or absence of carburization on the inner surface can be detected.
- FIG. 6 (a) is a diagram showing the result of examining the relationship between the detection signal and the carburization depth by extracting data of 4 ⁇ K ⁇ 8 from the data shown in FIG.
- data plotted with the same symbol is data having the same K value.
- the detection signal and the carburization depth show a relatively good correlation. Therefore, the carburization depth can be predicted to some extent from the magnitude of the detection signal.
- K 8
- the absolute value of the detection signal may be small (data surrounded by a dotted line in FIG. 6A), so that 4 ⁇ K as shown in FIG. 6B. ⁇ 6 is preferable.
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Abstract
Description
(1)被検査対象:内面に微細な浸炭を有する外径19mm、内径17mmの鋼管13本
(2)内挿コイル:外径16.5mm、長さ2mm、インピーダンス50Ω/100kHz
(3)励磁周波数(検査周波数):25kHz
上記試験のような内挿コイルを用いたきず検査は、きずによる電気抵抗変化を検知するものであり、一般的に高感度の検査が行われるため、磁性変動には敏感である。そして、磁性変動が生じている場合、X信号は磁性変動の大きさに応じた負の値になる(X軸の負の方向にデータがプロットされる)。しかしながら、図1に矢符A、B、Cで示すデータを除き、被検査対象の浸炭部位から得られたデータは正の値となり、浸炭を検知できない結果となった。図1に矢符A、B、Cで示すデータは負の値ではあるものの、最も絶対値の大きな負の値を示すデータ(矢符Aで示すデータ)であっても、巻き数6ターンの磁気テープから得られるデータとX信号の大きさは同程度であり、極めて微弱な磁性変動でしかない。
微細な磁性変動を検知するには、励磁コイルと検出コイルとを別体に設ける相互誘導法を採用することが好ましいものの、内挿コイルを用いる場合には、管内に挿入するコイル寸法の制約から、大きな励磁コイルを用いる相互誘導法を採用することが困難である。また、磁場強度を高めるには、大きな励磁電流を通電するために、励磁コイルの巻線径と、励磁コイルに励磁電流を供給する数10m程度の長さを有する電気ケーブルの径とを大きくすることが必要になるが、管の内径の制約を受ける。また、電気ケーブルの径を大きくしても、励磁電流を増加させると、励磁コイル自体の発熱が大きくなるため、検出コイルに温度変動が生じ、安定した検出信号(絶対値信号)を得ることが困難になるおそれもある。
さらに、内挿コイルを管内で走行させることになるため、高速走行が困難であると共に、管内に挿入した内挿コイルの引き戻しが必要となるため、管の製造ラインでの自動検査を行うには不向きである。
(1)励磁周波数(検査周波数):500Hz
(2)励磁電流:0.01A
(3)励磁コイルの巻き数:200ターン
(4)励磁コイルの長さ:70mm
そこで、本発明者らは、管の外面から当該管の内面における浸炭の有無を検知する方法において、微細な浸炭(微弱な磁性変動)の検知能力に対する励磁能力(磁場強度)と励磁周波数の影響について、以下のように更に鋭意検討を行った。
励磁コイルと検出コイルとを別体に設ける相互誘導法を採用する場合、磁場強度(励磁電流と単位長さ当たりの励磁コイルの巻き数との積)を大きくすると、これに応じて検出コイルに誘起される電圧も大きくなる。このため、検出コイルの出力信号を処理する信号処理部の感度(信号処理部が具備する増幅器のゲイン)を低くすることができ、電気的ノイズを抑制できる点で有利である。しかしながら、前述のように、磁性材の磁化特性はB-H曲線で表され、磁場強度が小さい場合の初透磁率は極めて小さく、磁場強度の増加に伴って透磁率が増加して最大値を示し、更に磁場強度を増加させると磁束密度が飽和して透磁率は小さくなる。このため、適正な磁場強度を与えなければ、微弱な磁性変動を検知することが困難となる。換言すれば、透磁率が小さい場合には、磁性変動に伴う検出コイルの出力信号(出力電圧)の変化が小さいため、微弱な磁性変動を検知できなくなる。これを信号処理部の感度を高めて補正する場合には、電気的ノイズが増加し、適正な検査ができなくなるおそれがある。
従って、微細な浸炭(微弱な磁性変動)の検知能力は、透磁率を最大化するという点で、励磁能力(磁場強度)に依存するといえる。
管の内面の浸炭により生じる磁性変動を管の外面から検知する場合、表皮効果の影響を軽減して浸透深さを深くするには、励磁周波数を低周波に設定する必要がある。一方、相互誘導法を採用する場合、励磁周波数を過度に低周波にすると、検出コイルに誘起される電圧が小さくなるため、検出コイルの出力信号を処理する信号処理部の感度(信号処理部が具備する増幅器のゲイン)を高める必要がある。このため、電気的なノイズが増加し、適正な検査ができなくなるおそれがある。
従って、微細な浸炭の検知能力は、励磁周波数に依存する。具体的には、浸透深さが励磁周波数の-1/2乗と概ね正の相関を有することと、信号処理部の感度(電気ノイズ)が励磁周波数と負の相関を有する(換言すれば、励磁周波数の-1乗と正の相関を有する)と考えられることから、微細な浸炭の検知能力は、励磁周波数の-3/2乗に依存することを見出した。
K=(I・N/L)・F-3/2 ・・・(1)
図2に示すように、パラメータKの値を増加させると、各磁気テープから得られる検出信号(X軸信号)の絶対値も増加し(換言すれば、浸炭検知能力が高まり)、両者は比較的良好な相関関係を有していることがわかる。この結果より、本発明者らは、パラメータKが浸炭検知能力の指標になり得ることを確認した。そして、本発明者らは、パラメータKの値を適切に調整することにより、微細な浸炭を検知できることを見出した。
すなわち、本発明は、以下の第1ステップ及び第2ステップを含むことを特徴とする。
(1)第1ステップ
管内面に浸炭の生じていることが既知である浸炭材を励磁コイル及び検出コイルに内挿させ、前記励磁コイルに通電する励磁電流の電流値をI(A)、前記励磁コイルの長さをL(mm)、前記励磁コイルの巻き数をN、前記励磁コイルに通電する励磁電流の周波数をF(kHz)とした場合に、前記検出コイルの出力信号に基づき前記浸炭材に生じている浸炭を検知できるように、下記の式(1)で表されるパラメータKの値を決定する。
K=(I・N/L)・F-3/2 ・・・(1)
(2)第2ステップ
前記決定したパラメータKの値が得られるように前記励磁コイルの条件を設定した後、被検査対象である管を前記励磁コイル及び前記検出コイルに内挿させ、前記検出コイルの出力信号に基づき前記管内面における浸炭の有無を検知する。
次に、本発明によれば、第2ステップにおいて、第1ステップで決定したパラメータKの値が得られるように励磁コイルの条件を設定した後、被検査対象である管内面における浸炭の有無が検知される。前述のように、第1ステップにおいて浸炭材の浸炭を検知できるようにパラメータKの値が決定されているため、このパラメータKの値が得られるように励磁コイルの条件を設定した後に被検査対象である管を検査すれば、当該管についても、パラメータKの値を決定するために用いた浸炭材に生じている浸炭と同等程度の浸炭を検知可能であることが期待できる。
すなわち、前記第2ステップにおいて、前記パラメータKの値が4≦K≦8を満足するように前記励磁コイルの条件を設定することが好ましい。
図3に示すように、本実施形態の渦流検査装置100は、検出センサ1と、信号処理部2とを備えている。図3において、検出センサ1は断面で図示されている。
まず、内面に浸炭の生じていない鋼管(以下、基準材という)を検出センサ1に挿入しない状態で、X信号及びY信号が0となるように(X信号及びY信号をそれぞれX軸成分及びY軸成分とするベクトルの先端に相当するスポットが図4に示すバランス点(原点)に位置するように)、増幅器22の前段に配置されたバランス回路(図示せず)のバランス量を調整して、同期検波器23から出力される第1信号成分及び第2信号成分をそれぞれ0とする。
次に、基準材を検出センサ1に挿入し停止させて、X信号が0で、Y信号が所定の電圧(例えば、4V)となるように(ベクトルの先端が図4に示す基準点に位置するように)、増幅器22の増幅率及び位相回転器24の位相回転量を調整する。
K=(I・N/L)・F-3/2 ・・・(1)
上記式(1)において、Iは励磁コイル11に通電する励磁電流の電流値(A)を、Lは励磁コイル11の長さ(mm)、Nは励磁コイル11の巻き数、Fは励磁コイル11に通電する励磁電流の周波数を(kHz)を意味する。
<試験条件>
(1)励磁電流の周波数F:0.3~1kHz
(2)励磁電流の電流値I:0.1~1A
(3)励磁コイルの長さL:70mm
(4)励磁コイルの巻き数N:200ターン
(5)浸炭材の材質:高Niオーステナイト系ステンレス鋼
(6)浸炭材の外径:φ15~25mm
(7)浸炭材の肉厚:0.9~1.25mm
(8)浸炭材の浸炭深さ:27~46μm
なお、4>Kのときには、励磁電流の電流値が小さいため、あるいは、励磁周波数が高く浸透深さが浅いため、浸炭材P0内面における磁場強度が小さくなる。その結果、浸炭材P0の透磁率が小さくなり、浸炭に伴う磁性変動を精度良く検知できない。一方、8<Kのときには、励磁電流の周波数が低周波であるため、浸透深さは深くなるものの、検出コイル12に誘起される電圧が低下して信号処理部2の感度(増幅器22のゲイン)が高くなる。このため、磁性変動に比べて導電率変動の影響が大きくなる。この結果、X信号が正の値になるものと考えられる。
2・・・信号処理部
11・・・励磁コイル
12・・・励磁コイル
21・・・発振器
22・・・増幅器
23・・・同期検波器
24・・・位相回転器
26・・・A/D変換器
27・・・判定部
100・・・渦流検査装置
P・・・鋼管
P0・・・浸炭材
Claims (2)
- 電磁気検査によって管内面における浸炭の有無を検知する方法であって、
管内面に浸炭の生じていることが既知である浸炭材を励磁コイル及び検出コイルに内挿させ、前記励磁コイルに通電する励磁電流の電流値をI(A)、前記励磁コイルの長さをL(mm)、前記励磁コイルの巻き数をN、前記励磁コイルに通電する励磁電流の周波数をF(kHz)とした場合に、前記検出コイルからの出力信号に基づき前記浸炭材に生じている浸炭を検知できるように、下記の式(1)で表されるパラメータKの値を決定する第1ステップと、
前記決定したパラメータKの値が得られるように前記励磁コイルの条件を設定した後、被検査対象である管を前記励磁コイル及び前記検出コイルに内挿させ、前記検出コイルからの出力信号に基づき前記管内面における浸炭の有無を検知する第2ステップと、
を含むことを特徴とする管内面の浸炭検知方法。
K=(I・N/L)・F-3/2 ・・・(1) - 前記第2ステップにおいて、前記パラメータKの値が4≦K≦8を満足するように前記励磁コイルの条件を設定することを特徴とする請求項1に記載の管内面の浸炭検知方法。
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